This invention relates generally to feedthrough terminal assemblies, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. More specifically, this invention relates to an improved feedthrough capacitor filtered terminal assembly of the type incorporating a hermetic seal to prevent passage or leakage of fluids through the terminal assembly, wherein breathable components are provided to accommodate and facilitate post manufacture and pre-usage testing of the hermetic seal.
Feedthrough terminal pin assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in active implantable medical devices such as cardiac pacemakers, defibrillators and the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage of electrical signals from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of patient body fluids into the medical device housing, where such body fluids could otherwise interfere with the operation of and/or cause damage to internal electronic components of the medical device.
In the past, two primary technologies have been employed to manufacture the hermetic seal. One technique involves the use of a ceramic insulator, typically alumina, which is sputtered to accept brazing material. This alumina insulator is brazed to the terminal pin or pins, and also to an outer metal ferrule of titanium or the like. The alumina insulator supports the terminal pin or pins in insulated spaced relation from the ferrule which is adapted for suitable mounting within an access opening formed in the housing of the medical device. In an alternative technique, the hermetic seal comprises a glass or glass-ceramic based seal forming a compression or matched glass seal for supporting the terminal pin or pins within an outer metal ferrule.
The feedthrough terminal pins are typically connected to one or more lead wires which, in the example of a cardiac pacemaker, sense signals from the patient's heart and also couple electronic pacing pulses from the medical device to the patient's heart. Unfortunately, these lead wires can act as an antenna to collect stray EMI signals for transmission via the terminal pins into the interior of the medical device. Such unwanted EMI signals can disrupt proper operation of the medical device, resulting in malfunction or failure. For example, it has been documented that stray EMI signals emanating from cellular telephones can inhibit pacemaker operation, resulting in asynchronous pacing, tracking and missed beats. To address this problem, hermetically sealed feedthrough filter terminal assemblies have been designed to include a filter capacitor for decoupling EMI signals in a manner preventing such unwanted signals from entering the housing of the implantable medical device. See, for example, U.S. Pat. Nos. 4,424,551; 5,333,095; 5,751,539; 5,905,627; 5,973,906; 6,008,980; and 6,566,978.
While feedthrough capacitor filter terminal assemblies have provided a significant advancement in the art, one potential area of concern is that the filter capacitor is often incorporated into the terminal pin assembly in a way that can mask a defective hermetic seal. More particularly, detection of a defective braze or a defective glass-based seal structure, which would permit undesirable leakage of patient body fluids when mounted on a medical device and implanted into a patient, can be obstructed by the mounting of the filter capacitor and its associated electromechanical connections. For example, with reference to the feedthrough filter capacitor shown in U.S. Pat. No. 4,424,551, a ceramic filter capacitor is bonded to a glass seal and then embedded in epoxy material. Typical post-manufacture leak testing is performed by mounting the feedthrough terminal assembly in a test fixture, and then subjecting one side of the feedthrough terminal assembly to a selected pressurized gas, such as helium. While the bulk permeability of the epoxy material is such that helium under pressure can penetrate therethrough in the presence of a defective hermetic seal, the duration of this pressure test (typically only a few seconds) is often insufficient to permit such penetration. Accordingly, the epoxy material can mask a defective hermetic seal. The thus-tested feedthrough terminal assembly can then mistakenly be incorporated into a medical device and implanted into a patient, wherein slow leakage of patient body fluids through the feedthrough assembly can cause the medical device to malfunction or fail.
One method to resolve this issue is depicted in FIGS. 1 and 2, which are similar to FIGS. 1 and 2 of U.S. Pat. No. 6,566,978, the contents of which are incorporated herein. These figures disclose a quadpolar feedthrough capacitor 20 mounted on a quadpolar feedthrough terminal assembly 22 and affixed to the ferrule 24 by means of non-porous electromechanical connection 26. The electromechanical connection material 26 around the entire perimeter of the capacitor 20 has bulk permeability insufficient to permit passage of helium gas (or other testing medium) during a leak detection test of standard duration. In order to facilitate leak detection testing in this prior art device, a gap 28 is formed between the ceramic capacitor 20 and the alumina hermetic seal insulator 30. The purpose of this gap 28 is to allow for ready passage of leak detection gases from the hermetic terminal areas or along lead wire 32 through the insulator 30 to flow to a leak detection vent hole 34. However, providing such a gap 28 between the ceramic capacitor 20 and the insulator 30 surface can result in the tendency to trap contaminants, cleaning solvents or the like into this enclosed space. Conductive polyimides or conductive epoxies or solders are typically used to form the electrical connection between the lead wire 32 and the inside diameter metallization 36 of the ceramic capacitor 20. Conductive polyimides or epoxies are also typically used to form the connection between the capacitor 20 outside diameter metallization 38 and the ferrule 24.
After curing, these conductive polyimide or epoxy materials are typically cleaned using a grit blasting system with sodium bicarbonate as the blasting medium. Sodium bicarbonate, otherwise known as baking soda, is highly soluble in water. Accordingly, de-ionized water rinses are used to ensure that no baking soda is left on the part as the sodium bicarbonate dissolves readily into the water cleaning solvent. After drying out, trace elements of the sodium bicarbonate can be left inside any cavity or air gap, for example, the gap 28 formed between the ceramic capacitor 20 and the alumina insulator 30. The sodium bicarbonate residue is hygroscopic. That is, it will tend to absorb moisture from the surrounding air which can degrade the electrical insulation properties of the quadpolar feedthrough terminal assembly 22.
For medical implant applications, it is typical that the insulation resistance requirement be 10 Gigaohms or even higher. In order to consistently achieve an insulation resistance greater than 10 Gigaohms, it is essential that all surfaces be extremely clean. Accordingly, any trace element of sodium bicarbonate or other contaminant left behind leads to rejection of the devices due to lowering of the insulation resistance below 10 Gigaohms.
Another issue associated with the gap 28 between the ceramic capacitor 20 and the insulator 30 is associated with the high voltage requirements of an implantable cardioverter defibrillator (ICD). Even low voltage devices like pacemakers are sometimes subjected to high voltage pulses. This is typical during an external defibrillation event. There has been a proliferation of automatic external defibrillators (AEDs) in the marketplace. One can now find AEDs in airplanes, hotels, sporting places and many other public venues. Accordingly, pacemaker wearers are being subjected to an ever-increasing number of high voltage shocks in the patient environment. One can see that the gap 28 is an area where electric field enhancement can occur. That is, when a high voltage is applied to the lead wires 32, there could be a tendency for a high electric field stress to occur in the air gap 28. These high electric field stresses can lead to ionization of the air gap 28, a resulting plasma, and a catastrophic high voltage breakdown of the assembly 20. This so called avalanche breakdown would cause an implantable medical device to not function, which would of course be life threatening for a pacemaker or defibrillator dependent patient.
Another method to resolve the issue of leak detection testing is depicted in FIGS. 3 and 4, which are similar to FIGS. 5 and 6 of U.S. Pat. No. 6,765,779, the contents of which are incorporated herein. FIGS. 3 and 4 disclose a unipolar feedthrough capacitor 40 mounted on a hermetic terminal assembly 42. The feedthrough capacitor 40 incorporates outer diameter metallization 44. An electrical attachment 46 is made from the capacitor outside diameter metallization 44 to the ferrule 48. This connection 46 is typically formed with a high temperature thermosetting conductive polymer such as a conductive polyimide. There are gaps left around the circumference of connection material 46 to provide helium leak detection pathways. This is generally described in U.S. Pat. No. 6,765,779 in column 2, lines 24 through 67 and in column 3 lines 1 through 33. There is also an axial gap 50 formed between the feedthrough capacitor 40 and the surface of the hermetic terminal 42. The purpose of this axial gap 50 is so that if there was a defective gold braze 52, 54, helium atoms could readily penetrate the annular space between the lead wire 56 and the inside diameter of the insulator 58. Accordingly, said helium atoms could then pass readily through the axial gap 50 and out through the spaces left in the circumferential conductive polyimide attachment material 46. As previously mentioned, leaving an axial gap 50 can trap contaminants between the capacitor 40 and the insulator 58 of the terminal assembly 42 and also has the tendency to enhance (squeeze or compress) electric fields during the application of a high voltage to the device.
Accordingly, there is a need for an EMI filtered hermetic feedthrough terminal assembly suitable for human implant that will avoid the issues associated with an air gap and/or leak detection pathways, but at the same time provide for a helium leak detection path. The present invention fulfills this need by providing an improved feedthrough terminal assembly suitable for use in an implantable medical device or the like, wherein the feedthrough assembly includes breathable electromechanical connections, breathable washers and breathable coatings and conformal coatings for accommodating and facilitating post-manufacture hermetic seal leak testing.